Insect pests are a major factor in the loss of the world's agricultural crops. For example, corn rootworm feeding damage or boll weevil damage can be economically devastating to agricultural producers. Insect pest-related crop loss from corn rootworm alone has reached one billion dollars a year.
Traditionally, the primary methods for impacting insect pest populations, such as corn rootworm populations, are crop rotation and the application of broad-spectrum synthetic chemical pesticides. However, consumers and government regulators alike are becoming increasingly concerned with the environmental hazards associated with the production and use of synthetic chemical pesticides. Because of such concerns, regulators have banned or limited the use of some of the more hazardous pesticides. Thus, there is substantial interest in developing alternatives to traditional chemical pesticides that present a lower risk of pollution and environmental hazards and provide a greater target specificity than is characteristic of traditional broad-spectrum chemical insecticides.
Certain species of microorganisms of the genus Bacillus are known to possess pesticidal activity against a broad range of insect pests including Lepidoptera, Diptera, Coleoptera, Hemiptera, and others. Bacillus thuringiensis and Bacillus papilliae are among the most successful biocontrol agents discovered to date. Insect pathogenicity has been attributed to strains of: B. larvae, B. lentimorbus, B. papilliae, B. sphaericus, B. thuringiensis (Harwook, ed. (1989) Bacillus (Plenum Press), p. 306) and B. cereus (International Publication No. WO 96/10083). Pesticidal activity appears to be concentrated in parasporal crystalline protein inclusions, although pesticidal proteins have also been isolated from the vegetative growth stage of Bacillus. Several genes encoding these pesticidal proteins have been isolated and characterized (see, for example, U.S. Pat. Nos. 5,366,892 and 5,840,868).
Microbial pesticides, particularly those obtained from Bacillus strains, have played an important role in agriculture as alternatives to chemical pest control. Pesticidal proteins isolated from strains of Bacillus thuringiensis, known as δ-endotoxins or Cry toxins, are initially produced in an inactive protoxin form. These protoxins are proteolytically converted into an active toxin through the action of proteases in the insect gut. See, Rukmini et al. (2000) Biochimie 82:109-116; Oppert (1999) Arch. Insect Biochem. Phys. 42:1-12 and Carroll et al. (1997) J. Invertebrate Pathology 70:41-49. Proteolytic activation of the toxin can include the removal of the N- and C-terminal peptides from the protein, as well as internal cleavage of the protein. Other proteases can degrade pesticidal proteins. See Oppert, ibid.; see also U.S. Pat. Nos. 6,057,491 and 6,339,491. Once activated, the Cry toxin binds with high affinity to receptors on epithelial cells in the insect gut, thereby creating leakage channels in the cell membrane, lysis of the insect gut, and subsequent insect death through starvation and septicemia. See, e.g., Li et al. (1991) Nature 353:815-821.
Recently, agricultural scientists have developed crop plants with enhanced insect resistance by genetically engineering crop plants to produce pesticidal proteins from Bacillus. For example, corn and cotton plants genetically engineered to produce Cry toxins (see, e.g., Aronson (2002) Cell Mol. Life. Sci. 59(3):417-425; Schnepf et al. (1998) Microbiol. Mol. Biol. Rev. 62(3):775-806) are now widely used in American agriculture and have provided the farmer with an environmentally friendly alternative to traditional insect-control methods. In addition, potatoes genetically engineered to contain pesticidal Cry toxins have been sold to the American farmer. The presence of endogenous proteases in plants that can degrade and inactivate the insect toxins expressed in these transgenic plants, however, limits the usefulness of these pest-control techniques.
Researchers have determined that plants express a variety of proteases, including serine and cysteine proteases. See, e.g., Goodfellow et al. (1993) Plant Physiol. 101:415-419; Pechan et al. (1999) Plant Mol. Biol. 40:111-119; Lid et al. (2002) Proc. Nat. Acad. Sci. USA 99:5460-5465. Previous research has also shown that insect gut proteases include cathepsins, such as cathepsin B- and L-like proteinases. See, Shiba et al. (2001) Arch. Biochem. Biophys. 390:28-34; Purcell et al. (1992) Insect Biochem. Mol. Biol. 22:41-47. For example, cathepsin L-like digestive cysteine proteinases are found in the larval midgut of Western corn rootworm. See, Koiwa et al. (2000) FEBS Letters 471:67-70; Koiwa et al. (2000) Analytical Biochemistry 282: 153-155.
While investigators have previously genetically engineered plants, particularly crop plants, to contain biologically active (i.e., pesticidal) Cry toxins, these foreign proteins can be degraded and inactivated by proteases present in these transgenic plants. Moreover, researchers to date have not effectively utilized the protoxin forms of pesticidal polypeptides in conjunction with endogenous plant or insect gut proteases to control plant pests. Thus, new strategies for modifying insect toxins and utilizing these modified insect toxins in pest management strategies are desired.